Cosmology of axion dark matter (2403.17697v2)

Abstract: I present an introduction and topical review on axions as a dark matter candidate. Emphasis is placed on issues surrounding the cosmology of axion dark matter that are relevant for present-day searches, including: early-Universe production mechanisms, predictions of the axion mass, bounds on axion properties derived from cosmological data, as well as the direct and indirect detection of relic axion populations.

• The paper provides a detailed analysis of axions produced via the misalignment mechanism as a dark matter candidate.
• The paper discusses cosmological constraints, showing how CMB and large-scale structure observations limit axion properties.
• The paper evaluates current and proposed experimental approaches aimed at detecting axion signals in strong magnetic fields.

Essay on "Cosmology of Axion Dark Matter"

The paper "Cosmology of Axion Dark Matter" by Ciaran A. J. O'Hare offers a comprehensive examination of axions as a candidate for dark matter, with a particular focus on cosmological implications. This piece provides a detailed exploration of the mechanisms by which axions could constitute dark matter, the constraints imposed by cosmological observations, and the current status of experimental searches.

Axions and Dark Matter

Axions were initially proposed as a solution to the Strong CP problem in quantum chromodynamics (QCD), representing a light, neutral pseudoscalar particle. Over time, the axion has emerged as a viable dark matter candidate due to its unique properties and production mechanisms in the early Universe. The paper explores these mechanisms, notably the misalignment mechanism, which posits that axions were produced through non-thermal processes in the early Universe. This mechanism involves axions starting from a misaligned field value and later transitioning into coherent oscillations that behave as cold dark matter.

Cosmological Constraints

O'Hare discusses cosmological bounds on the axion’s properties, particularly its mass, which is a crucial parameter in determining its viability as dark matter. The paper explains that cosmological observations, such as the Cosmic Microwave Background (CMB) and large-scale structure, impose significant constraints on the axion parameter space. Specifically, these observations require that axions, if they are to make up the entirety of dark matter, must not significantly alter the well-established structure formation process and the expansion history of the Universe.

A notable part of the discussion involves the axion’s contribution to the effective number of relativistic species, $N_{\rm eff}$, during the early Universe. If axions were in thermal equilibrium with standard model particles, they would contribute to $N_{\rm eff}$, affecting the CMB power spectrum. Current data do not favor additional contributions beyond those from known particles, thus constraining the axion’s coupling and mass.

Theoretical Implications and Models

The theoretical implications of axion dark matter are also explored, with insights into different models like the QCD axion and axion-like particles (ALPs). The paper elaborates on how variations in the underlying theoretical model affect axion properties and their detection prospects. The QCD axion's mass is tied to the Peccei-Quinn scale, whereas ALPs allow for a broader parameter space as they are not necessarily bound by the same constraints as QCD axions.

Experimental Searches and Future Prospects

Current and future experimental searches for axion dark matter form a crucial part of O'Hare's review. Various experimental approaches are discussed, including haloscopes that aim to detect the axion-photon conversion in a strong magnetic field. The constraints from these experiments, in conjunction with astrophysical observations, delineate a promising yet challenging path forward.

The paper also highlights areas where future developments could emerge. Enhancements in astrophysical observations, improvements in experimental design, and deeper understanding of the theoretical landscape all hold promise for advancing the field. In the coming years, precision measurements of cosmological parameters and new data from next-generation experiments could provide either constraints or potential signals of axions.

Conclusion

Ciaran O'Hare's review offers a detailed and thorough look into the cosmology of axion dark matter, highlighting both the promise and challenges of axions as a dark matter candidate. The interplay between theoretical predictions, cosmological constraints, and experimental detections presents a rich landscape for current and future exploration. As research continues, the axion hypothesis remains a compelling avenue for unraveling the mysteries of dark matter, with significant implications for both particle physics and cosmology.